The use of powder metallurgy (P/M) is known for forming a variety of net-shape parts that otherwise typically require fabrication by such conventional methods as stamping, machining or die casting. Examples of P/M processes include the molding of metal powders to form structural parts such as gears, frames and wheels, and the molding of iron and iron alloy powders to form magnets, including soft magnetic cores for transformers, inductors, AC and DC motors, generators, and relays. A significant advantage to using powdered metals is that intricate part configurations can be produced without the need to perform additional machining and piercing operations. As a result, the formed part is often substantially ready for use immediately after the forming operation. Various compaction methods are known, including compression molding, injection molding and extrusion, and post-molding operations such as annealing and sintering can often be used to improve mechanical and physical properties, depending on the composition of the powder and the requirements of the application.
P/M compositions can also be tailored to facilitate mixing and compaction of the powder materials and promote the properties of the end P/M product. For example, metallurgical additives and inorganic materials may be added to promote the physical and mechanical properties desired for the product, and lubricants may be mixed with the powder to increase the density achievable during compaction. In situations where a permanent binder is desired, such as AC electromagnetic cores, processes and materials have been developed by which a thin encapsulating layer of a polymer is coated on the powder particles, which are then compression molded to bind the particles together with the polymer. Binders also serve to increase green strength and reduce the incidence of cracking when the compacted article is removed from the die. For this reason, binders have also been employed as temporary additives to powder mixtures. As an example, in structural and DC electromagnetic applications, a binder may be employed to promote green strength in the as-molded article, and later removed by heating. The article can then undergo annealing and sintering as may be necessary to obtain the final properties desired for the article.
Solid additives such as binders and lubricants are often mechanically blended with a dry metal powder, after which the resulting mixture is uniaxially molded into a net-shape part. After molding, the part is heated in a suitable atmosphere to a temperature and for a duration sufficient to remove the lubricant (delube) and then, if appropriate, remove the binder and sinter (fuse) the metal powder together. Binders and lubricants can also be solution blended with metal powders, by which these additives are added in a slurry mixer with a tackifier dissolved in a solvent. Solution blending with a tackifier is employed to reduce dusting and segregation during mixing, and to adhere the solid additives to the metal powder particles. In typical solution-blending methods, the binder and lubricant are insoluble in the solvent, and remain as solid particles in the metal powder slurry created as a result of the dissolved tackifier. As the slurry is mixed, the solvent is evaporated and the binder and lubricant particles are adhered to the metal particles with the tackifier. Similar to articles formed by powders mechanically-blended with binders and lubricants, solution-blended powders are typically heated under appropriate conditions to remove the lubricant and, if desired, the binder, and may be further heated to sinter the metal powder. Articles formed by solution-blending techniques typically exhibit improved green density and green strength over those formed from mechanically-blended powders.
In practice, tackifiers used in solution-blending processes serve only a temporary role to adhere binders, lubricants and/or any other insoluble additives to the metal particles. Any residues of the tackifier that remain within the metal powder can be detrimental to the properties of the powder and resulting article. For example, tackifier residues can degrade the flowability of the powder when introduced into the die, degrade the compressibility of the powder, lower the density of the compacted article, and increase the force required to eject the article from the die. Accordingly, it would be desirable if the advantages of solution-blended powders could be achieved without the use of a discrete adhesion-promoting additive that is detrimental to the P/M process or product.